Tooth Mg/Ca ratios and Aristotle's lantern morphometrics reflect trophic types in echinoids

Several studies have inferred the ecological significance regarding the morphometrics of Aristotle's lantern and the mechanical properties of magnesium in echinoid teeth. This study attempts to combine these aspects, connecting them to the trophic habits of three native and an invasive echinoid in the Eastern Mediterranean Sea. Spatiotemporal data from the central and southern Aegean Sea were obtained, regarding the relative size of lanterns and demi-pyramids of Arbacia lixula, Paracentrotus lividus, Sphaerechinus granularis, and the invasive echinoid Diadema setosum and the Mg/Ca ratios of four zones on the tooth cross-section. Since environmental factors affect the examined factors, data for temperature, salinity, and concentration of chlorophyll-a were included in a principal component analysis. A. lixula and P. lividus presented intraspecific differences in the relative size of the lantern and demi-pyramid, while S. granularis and D. setosum exhibited variation in the elongation index. Differences in the Mg/Ca ratios were observed for all species although in different zones. Temperature appears to be related to all Mg/Ca zones except for the stone part, while the elongation index showed an inverse trend to all other morphometric parameters. The results of the PCA for the four species on the spatiotemporal level exhibited a distinction of individuals with season but not species, except for A. lixula, an omnivore with a carnivorous tendency, which was clearly separated from the herbivorous species. Using hierarchical clustering on the principal components it was evident that the three native species occupy different clusters, but when D. setosum was added, it shared the same cluster with S. granularis. This might infer similar feeding preferences, specifically for coralline algae, which might lead to a swift in the ecological equilibrium in regions, where D. setosum is found, either by affecting habitat type, or by restricting the distribution of S. granularis as was previously observed with Diadema africanum.